When two radio devices are placed in proximity they can interfere, which leads to a degradation of the quality of the communications. More specifically, the signal transmitted by a transmitter, also called a “direct” signal or “interfering” signal, is superimposed on the “wanted” signal received by a receiver.
Due to the proximity between the two devices the direct signal is very intense compared to the wanted signal, which contains the data which must be received by the receiver.
The interference between two devices is due principally to two effects.
Due to the fact that the signal transmitted by the transmitter is very intense in the receiver's antenna its amplifiers may be saturated. This effect, which is also known as desensitisation of the receiver, may occur even if the carrier of the transmitter device is outside the receiver band of the receiver device.
The second effect is due to the broadband noise which is associated with the direct signal. This broadband noise may be due, in particular, to the non-linearities of the various modules present in the transmitter. This broadband noise may cover, totally or partially, the frequency receiver band of the second device.
The interference is particularly substantial when the frequency band of the transmitter is close to the frequency band of the receiver, or when both frequency bands are coincidental.
One of the solutions known to those skilled in the art to reduce the intensity of the interfering signal is to use radio-frequency filters, or RF filters.
Nevertheless, these solutions prove to be impractical due to the fact that it is necessary to choose specific RF filters for each transmitter/receiver band, or non-applicable when the transmission and reception use the same frequency band. In addition, RF filters produced insertion losses which can reduce the power of the transmitted signal when they are used in transmitters, and the sensitivity of the receiver device when they are used in receivers.
Other solutions known to those skilled in the art relate to the physical elimination of the interference by subtraction or compensation of the interfering signal. These methods are based on an analogue subtraction of a reference signal from the received signal in order to recover the wanted signal exclusively.
The reference signal can be obtained by making an analogue copy of the transmitted signal, and by transmitting its amplitude and phase using an analogue processing circuit.
This analogue correction process is very effective to eliminate the effects of desensitisation of the receiver, and enables saturation of the receiver device to be prevented. However, this analogue correction process is effective only if the analogue copy of the transmitted signal is sufficiently well time-synchronised with the interference which must be eliminated. In addition, the solutions known to those skilled in the art do not enable allowance to be made for the distortions of the reference signal introduced by the analogue processing circuit.
Certain systems, also known to those skilled in the art, enable digital processing of the interference to be accomplished in the band of the receiver device. In a general sense this processing enables the signals to be combined by accomplishing more complex operations which would not be possible with analogue processing.
In this case the reference signal is a digital signal taken in the transmitter device before being converted into analogue format, an operation which occurs prior to transmission.
However, since the digital reference signal is taken before it is converted into an analogue signal, these solutions are unable to take into account the non-linearities of the amplification chain of the transmitter device. These non-linearities are at least partly responsible for the broadband noise observed in the reception channel.